**2. Different polymeric NPs designed as a VADS**

Polymeric NPs, namely NPs made of highly biocompatible polymers, such as polystyrene, PLGA, proteins, chitosan, and hyaluronic acid (HA), have recently been widely explored as a DDS (drug delivery system) as well as VADS and show many excellent properties beneficial for therapeutic delivery of agents, for example, high stability can shelter the loaded Ags from environmental detriment and in vivo unwanted degradation; biocompatibility can reduce toxicity to recipients and their compliance; ease in modulation of particle size, surface charge, and specific binding characteristics allow developing multifunctional VADS [3, 4]. Thus, through fulfilling multiple functions, polymeric NPs are able to improve the efficacy of vaccines, for example, they can form a depot to enhance vaccine efficacy via elongated release and exposure of Ag at the site of injection; they can targetedly deliver vaccines to APCs promoting cellular uptake of Ags and thus enhancing Ag stimulation efficiency; they can alter intracellular process of Ags adapting immune responses toward the beneficial humoral and/ or cellular pathways; and also they can provide diverse administration routes for vaccination to elicit the desired immunity at circulation system as well as mucosal sites [6, 11].
